Determining the position of unmanned underwater vehicles (UUV's) or autonomous underwater vehicles (AUV's) is a difficult problem as high frequency radio waves generally do not travel significant distances in water. This renders direct reception of GPS signals, which would otherwise provide good positional fixes, generally unusable. Conventional methods for determining the position of an underwater vehicle typically involve surfacing the vehicle to take a GPS fix, and then going underwater and using various sensors (Doppler velocity logs, Inertial guidance INS, ranging from acoustic buoys) and dead reckoning calculations to determine movement from the fixed position. This process both exposes the vehicle to the surface (taking it away from its mission and potentially exposing it to danger) and results in errors in positioning as errors can accumulate from the dead reckoning calculations.
There are also several schemes disclosed in the prior art that allow a buoy float to be sent to the surface from an underwater vehicle from a tether to obtain a GPS fix. These include U.S. Pat. No. 6,058,874; U.S. Pat. No. 5,379,034; U.S. Pat. No. 5,291,194; U.S. Pat. No. 5,377,165; and U.S. Pat. No. 6,907,839. Each of these patents disclose a scheme that brings some float attached by a wire up to the surface to allow a GPS and/or RF link to operate while the vehicle is below the surface. One problem with these methods is that they do not determine the position of the vehicle, but generally the position of the float. These references thus fail to address the error introduced by the offset of the float from the vehicle by its tether. In addition, longer length tethers are not described in the art. If a GPS accuracy of less than one meter is provided, and a tether used to attach the GPS float to the vehicle has a length of 10 meters, that introduces an order of magnitude error increase in the accuracy of the position fix. This becomes very important when the UUV is being used for operations such as hydrographic surveys, where a 10 meter overall accuracy is required, or mine reconnaissance and removal, where the vehicle may have to go back to the same locations several times.